Instrumentation for sensing moisture content of material using a transient thermal pulse

Abstract

Instrumentation for sensing moisture content of material using a transient thermal pulse is comprised of a sensing probe (12) having a sensing element (14) in the form of a ribbon excited by a constant current pulse from a source (16) to increase the temperature, and therefore the resistance, of the ribbon linearly. Moisture in web material (10) will limit the increase of temperature during the pulse in proportion to the moisture content. This increase in temperature produces a proportional increase in resistivity which is measured with a Wheatstone bridge (R 1 , R 2 , R 3 , 14) as a change in voltage displayed by a measurement display unit (20). The probe (12) is glued in a shallow groove of a Lucite bar (15c) and connected to copper pins (15a, 15b) embedded in the bar. A cylindrical Lucite block (21) may be used to hold the sensing element (14) connected to axial pins (22a, 22b) in order for the probe to roll and thus reduce its wear.

What is claimed is: 
     
       1. A transient method of measuring the moisture content of material using a short current pulse through a sensor having an element that is thermally and electrically conductive, the resistance of which varies as a function of its temperature, comprised of determining the resistance of said element on application of a short pulse of constant current energy, placing said element in thermal contact with said material, heating said element with said short pulse of constant current energy, determining the peak change in resistance of said element during the application of said short pulse of current while said element is in contact with said material and always previous to any thermal equilibrium that would be reached by said element while in contact with said material, said peak change being a measure of moisture content of said material. 
     
     
       2. A transient method of measuring the moisture content of material using a short current pulse through a thermally and electrically conductive sensor element the resistance of which varies in proportion to its temperature, comprising the steps of measuring the resistance of said sensing element at ambient temperature,   placing said element in thermal contact with said material,   heating said element with constant current pulse, thereby to raise its temperature, said constant current pulse having a duration in the order of milliseconds,   determining the resistance of said element at the peak of its raised temperature during the presence of said constant current pulse and always previous to any thermal equilibrium that would be reached by said element while in contact with said material, and   correlating the measured resistances of said element with moisture content of said material.   
     
     
       3. A method as defined in claim 2 wherein the step of correlating the measured resistance is by use of a graph of experimental data for said material over a desired range of moisture content. 
     
     
       4. A method as defined in claim 3 wherein resistance is determined as a voltage across said element while said constant current pulse is being applied, and said experimental data is a graph of change in peak voltage as a function of moisture content. 
     
     
       5. A method as defined in claim 1 wherein said element is periodically pulsed with constant current pulses, and resistance of said element during each pulse period is determined as a voltage across said element, whereby an alternating current signal is derived from said element having an amplitude proportional to moisture content for continually monitoring the moisture content of said material. 
     
     
       6. Apparatus for sensing the moisture content of material using a short current pulse comprising a sensor adopted to be in thermal contact with said material, said sensor having a thermally and electrically conductive element the resistance of which varies in proportion to its temperature,   means for pulsing said element at the time the moisture content is to be sensed with a constant current pulse, said pulse having a duration in the order of milliseconds, and   means for determining said change in resistance achieved in said element in response to each pulse of constant current, always previous to reaching any thermal equilibrium as a measure of moisture content of said material.   
     
     
       7. Apparatus as defined in claim 6 wherein said sensor is comprised of a thin ribbon of metal and means for placing said ribbon in thermal contact with said material. 
     
     
       8. Apparatus as defined in claim 7 wherein said means for placing said ribbon in thermal contact with said material is comprised of a body of material that is not an electrical conductor and is a poor thermal conductor. 
     
     
       9. Apparatus as defined in claim 8 wherein said body is a bar and said ribbon is mounted in a groove on one face of said body flush with the surface thereof, and electrical conductors are connected to ends of said ribbon through said body. 
     
     
       10. Apparatus as defined in claim 8 wherein said body is a cylinder and said ribbon is mounted in a groove parallel to the axis of said cylindrical body flush with the surface thereof, and electrical conductors are connected to ends of said ribbon through axial rotary connectors extending into ends of said body, whereby said body may roll on said material in response to relative motion between said body and said material the moisture content of which is being measured.